The present invention relates to semiconductor memory devices and, more particularly, to multi-level semiconductor memory devices.
Nonvolatile memories using resistance materials include phase-change random access memories (PRAMs), resistive RAMs (RRAMs), and magnetic RAMs (MRAMs).
While dynamic RAMs (DRAMs) or flash memories store data using charges, nonvolatile memories using resistance materials store data using a state change of a phase-change material such as chalcogenide alloy (in the case of PRAMs), a resistance change of a variable resistance material (in the case of RRAMs), or a resistance change of a magnetic tunnel junction (MTJ) thin film according to a magnetization state of a ferromagnetic material (in the case of MRAMs).
Various methods for storing as many bits as possible within a limited area of a semiconductor wafer have been developed. For example, one possible way of storing as many bits as possible within a limited area of a wafer is to develop and use sophisticated lithographic methods and apparatuses in order to increase the number of memory cells that are formed on the wafer. Another way of improving the integration of a memory device is to store more than one bit per unit area in a memory cell. Such a memory device is commonly referred to as a multi-level memory device. For example, in a multi-level memory device that uses a resistance material, each memory cell may include at least three different resistance levels according to data stored. However, in a case of using a multi-level phase change material and a multi-level electrode for the purpose of implementing the resistance levels, the memory device may have a complicated structure, and the feature size of a chip may increase.